The present invention relates to a surface acoustic wave transducer for use in a surface acoustic wave device such as a SAW filter and a SAW resonator, and more particularly to a surface acoustic wave transducer comprising a piezoelectric substrate having a natural single-phase unidirectional transducer (termed as NSPUDT) property and an electrode structure having a directivity. The present invention also relates to a surface acoustic wave filter having such a transducer.
Heretofore, there has been widely used a transversal type SAW filter, in which on a surface of a piezoelectric substrate, are arranged a transmitter side transducer including interdigitally arranged positive and negative electrodes to be connected to two output terminals of a single phase signal source having a phase difference of 180xc2x0, and a receiver side transducer also including interdigitally arranged positive and negative electrodes, said transmitter and receiver side transducers being separated by a given distance so as to extract a specific frequency.
In such a SAW filter, it is required to reduce an insertion loss as well as to suppress a ripple within a given frequency band. Since a conventional interdigital electrode structure shows the bidirectionality, a theoretical insertion loss amounts to 6 dB and thus the insertion loss could not be suppressed any more. Furthermore, in order to improve a performance of the surface acoustic wave device, not only the insertion loss has to be decreased, but also it is also important to flatten a phase characteristic and to improve a frequency characteristic such as a suppression of ripple within a pass band and a suppressed band.
In order to satisfy the above mentioned requirements, there has been used a unidirectional transducer, in which an insertion loss can be suppressed to a value not larger than 1 dB and good phase and frequency characteristics can be attained. For the unidirectional transducer, there have been proposed various kinds of types, which may be roughly classified into (a) multi-phase type unidirectional transducer and (b) single-phase type unidirectional transducer. Further, for the latter single-phase unidirectional transducer, there have been proposed a single-phase unidirectional transducer utilizing an internal reflection due to asymmetry of an electrode structure and mass load effect, a single-phase unidirectional transducer utilizing a reflection by a floating electrode, and a natural single-phase unidirectional transducer utilizing an anisotropy of a substrate. In surface acoustic wave devices using such unidirectional transducers, the directivity is attained by a fact that a phase difference between an exiting wave and an excited wave and a reflected wave have the same phase viewed in a propagating direction, but have opposite phased viewed in a direction opposite to the propagating direction.
In the above mentioned single-phase unidirectional transducers other than the natural single-phase unidirectional transducer, the electrode structure is complicated, and particularly an edge distance and a width of electrode fingers have to be smaller than xcex/4. In accordance with an increase in an operating frequency, said dimensions become extremely small, and it is difficult to manufacture accurately the electrodes having the desired dimensions.
As one solution for solving such a problem, there has been proposed a natural single-phase unidirectional transducer (NSPUDT), in which due to the anisotropy of the substrate itself, the unidirectionality can be attained even by using a conventional electrode, in which the edge distance and width of electrodes are set to xcex/4. In this surface acoustic wave device utilizing the NSPUDT behavior, the anisotropy of the substrate itself is utilized. Quartz substrate, LiNbO3 substrate and LiTaO3 substrate have been known as such a piezoelectric substrate showing the unidirectionality due to the anisotropy. The inventors have further recognized that a lithium tetraborate substrate of a special cut shows the NSPUDT property. Particularly, upon being compared with the other substrates showing the natural single-phase unidirectional transducer property, this lithium tetraborate substrate could provide an ideal surface acoustic wave device due to a fact that it has a large electromechanical coupling constant K2, a zero delay time temperature coefficient, a zero power flow angle and so on.
However, in the surface acoustic wave device utilizing the above mentioned NSPUDT, since the anisotropy of the substrate itself is utilized, there is a problem that it is difficult to obtain unidirectional transducers whose forward directions are opposed to each other, one for the output side, i.e. transmitter side transducer and the other for the input side, i.e. receiver side transducer. In order to solve this problem, there have been proposed various methods for reversing the directivity. For instance, the directivity of one of the transmitter and receiver side transducers may be reversed by using electrodes made of a material whose phase of reflection coefficient is different from that of the other transducer by 180xc2x0, or by providing recesses in a substrate surface and embedding electrodes in the recesses.
However, according to the generally used manufacturing process, the formation of electrodes from different electrode materials and the embedded electrodes might cause the complication in the designing and manufacturing and high cost, and thus it would be difficult to obtain a desired accuracy, so that the desired frequency and phase characteristics could not be attained.
Moreover, if a transversal type surface acoustic wave filter for extracting a signal within a specific frequency band is realized by providing transmitter and receiver side transducers on the substrate having the anisotropy, when the electrode structure of the transmitter side transducer is made of a different material than that of the receiver side transducer, a propagating velocity of a surface acoustic wave in the transmitter side transducer might be different from that in the receiver side transducer and a center frequency might shift between the transmitter side transducer and the receiver side transducer.
Further, in case of using the NSPUDT substrate having a large reflection coefficient of electrode fingers, the reflecting effect of the electrodes fingers might become too strong to control a transduction characteristic.
The present invention has for its object to provide a surface acoustic wave transducer and a surface acoustic wave filter, in which the above mentioned drawbacks of the known surface acoustic wave transducer having the NSPUDT property can be removed or mitigated, and superior frequency and phase characteristics can be obtained by a simple manufacturing process.
According to the invention, a surface acoustic wave transducer comprising an anisotropic piezoelectric substrate being cut to have a natural single-phase unidirectional transducer property and at least one transducer structure having an exciting electrode structure and a reflector structure formed on said substrate, characterized in that when xcex is a wavelength of a fundamental surface acoustic wave, said exciting electrode structure includes a positive electrode having a plurality of electrode fingers arranged at a pitch xcex and a negative electrode having at least one electrode finger interdigitally arranged between said electrode fingers of the positive electrode with a center distance of xcex/2, that said reflector transducer includes a plurality of electrode fingers arranged with a center distance of xcex/2, and that a distance Lg between said exciting electrode structure and the reflector structure is set to Lg=(2n+1)xcex/4 (n being a positive integer).
In a conventional piezoelectric substrate without the anisotropy, a reflection center may be situated at a center of an electrode finger although a reflection occurs at input and output side edges of the electrode finger. Contrary to this, in the piezoelectric substrate being cut to have the natural single-phase unidirectional transducer behavior, a phase rotation of xc2x145xc2x0 is added by the reflection. Due to this 45xc2x0 phase rotation, the reflection center is subjected to a phase shift of xcex/8 relative to the center of the electrode finger in a positive or negative direction of the operating direction determined by the NSPUDT property of the substrate. The present invention can realize the unidirectional transducer having the directivity which is opposite to the operating direction due to the NSPUDT property of the substrate by suitably utilizing the anisotropy of the substrate, the reflection effect of the electrode and the reflection effect of the reflector structure. For instance, in case of using a STX+25xc2x0 cut quartz substrate, the 45xc2x0 phase rotation is induced by the reflection and the reflection center is shifted by xcex/8 in a positive direction of the operating direction determined by the NSPUDT property of the substrate. Therefore, when a distance between the exciting electrode structure and the reflector structure (a center distance between an electrode finger of the exciting electrode structure and an electrode finger of the reflector structure, said electrode fingers being adjacent to each other) is set to a value of (2n+1)xcex/4 (n being a positive integer), a surface acoustic wave propagating in the forward direction toward the reflector structure, reflected by the reflector structure and propagating in the backward direction (the operating direction determined by the NSPUDT property) becomes in-phase with respect to a surface acoustic wave emitted by the exciting electrode structure in the backward direction, and these surface acoustic waves intensify each other. This results in the transducer having the reversed directivity. In this transducer, a reflecting effect becomes a sum of the reflection effect by the electrode finger of the exciting electrode structure and the reflection effect by the electrode finger of the reflector structure. Therefore, by suitably adjusting the numbers of the electrode fingers of the exciting electrode structure and reflector structure, it is possible to realize the transducer having the directivity reversed with respect to the operating direction due to the NSPUDT behavior and the transducer having the non-reversed directivity.
According to the invention, the electrode fingers of the exciting electrode structure and reflector structure are arranged periodically at a pitch of xcex/2, and the reflection effect by the reflector structure itself is suitably utilized. It should be noted that there is also a known transducer having the exciting electrode and reflector arranged on a bidirectional substrate without the anisotropy. However, this known transducer differs inherently from the present invention in that the phase of the reflected wave is shifted by deviating the reflector with respect to the exciting electrode. That is to say, in the known transducer of the reflection bank type, the electrode fingers of the reflector are periodically shifted with respect to the electrode fingers of the exciting electrode by xcex/2. If this known arrangement is applied to the anisotropic substrate having the NSPUDT behavior, it would be impossible to utilize the reflection effect efficiently to realize a practical transducer.
Furthermore, in case of using an anisotropic substrate such as Li2B4O7 substrate having a cut angle, represented by Euler""s angle, of xcfx86=0xc2x0, xcex8=51xc2x0 and xcfx86=90xc2x0, in said substrate a phase rotation of xe2x88x9245xc2x0 being produced by the reflection, since the phase shift of xcex/8 is introduced in a direction opposite to the orientation of the substrate, when a distance between the exciting electrode structure and the reflector structure is set to (2n+1)xcex/4 (n being a positive integer), a surface acoustic wave emitted by the exciting electrode structure in a direction opposite to the operating direction of the NSPUDT property has same phase with a surface acoustic wave reflected by the reflector structure. In this manner, it is also possible to realize the transducer having the reversed directivity. In case of forming the transversal type surface acoustic wave filter by providing the transmitter side transducer and receiver side transducer on a piezoelectric substrate, both the transmitter and receiver side transducers may be formed by one and same electrode material, and thus a manufacturing process becomes much simpler.